Lighting circuit

Abstract

A lighting circuit includes a DC/DC converting circuit for boosting an input DC voltage so as to produce boosted DC power, a detecting circuit for producing a detection signal in response to the output voltage of the DC/DC converting circuit, a DC/AC converting circuit for producing AC power to be supplied to the discharge tube from the boosted DC power, a starting circuit for starting up the discharge tube, and a control circuit for receiving the detection signal from the detecting circuit. The control circuit contains a capacitor which is charged based upon the detection signal, a first discharging unit for discharging the capacitor, a second discharging unit for providing a discharging path for the capacitor in response to turning-OFF of the input DC voltage, and a signal producing unit for producing a control signal used to adjust the output of the DC/DC converting circuit in response to a terminal voltage of the capacitor.

1. A lighting circuit which lights a vehicle discharge tube, the lighting circuit comprising:
 a DC/DC converting circuit which boosts an input DC voltage so as to produce boosted DC power; 
 a detecting circuit coupled to said DC/DC converting circuit, which produces a detection signal in response to an output voltage of said DC/DC converting circuit; 
 a DC/AC converting circuit which receives said boosted DC power and produces AC power to be supplied to said discharge tube; 
 a starting circuit coupled to said DC/AC converting circuit, which starts up said discharge tube; and 
 a control circuit coupled to said DC/DC converting circuit, which receives said detection signal from said detecting circuit, 
 wherein 
 said control circuit includes: 
 a difference signal producing unit which produces a difference signal indicative of a difference between said detection signal and an initial value of said detection signal; 
 a charging unit which receives said difference signal and supplies an electric charge in a case where said difference is greater than or equal to a predetermined threshold value; 
 a capacitance element coupled to an output of said charging unit via a resistive circuit; 
 a first discharging unit which is coupled to the output of said charging unit and to said capacitance element via said resistive circuit; 
 a second discharging unit coupled to said capacitance element, which provides a discharging path for said capacitance element in response to a turning-OFF of said input DC voltage; and 
 a signal producing unit which produces a control signal used to adjust the output of said DC/DC converting circuit in response to a terminal voltage of said capacitance element. 
 
   
   
     2. A lighting circuit as claimed in  claim 1 , wherein said second discharging unit includes:
 a diode having an anode coupled to said capacitance element; 
 a first resistor element coupled between a cathode of said diode and a first power supply line; and 
 a second resistor element coupled between the cathode of said diode and a second power supply line. 
 
   
   
     3. A lighting circuit as claimed in  claim 1 , wherein said second discharging unit includes:
 a diode having an anode coupled to said capacitance element; and 
 a resistor element coupled between a cathode of said diode and a power supply line. 
 
   
   
     4. A lighting circuit as claimed in  claim 1 , wherein
 said second discharging unit includes: 
 a diode having an anode coupled to said capacitance element and a cathode coupled to a first power supply line; and 
 a resistor element coupled between the cathode of said diode and a second power supply line. 
 
   
   
     5. A lighting circuit as claimed in  claim 4 , further comprising:
 an interrupting circuit interposed between said first power supply line and the cathode of said diode, which interrupts a connection between said first power supply line and the cathode of said diode when said input DC voltage is turned OFF.